Various types of solar energy collectors are known, and include non-concentrating types and concentrating types. Non-concentrating types intercept parallel unconcentrated rays of the sun with an array of detection or receiving devices such as a solar panel of photovoltaic cells or hot water pipes, for example. Photovoltaic cells use photovoltaic material to absorb sunlight and convert it directly to electricity. A concentrating type collector focuses energy rays using a special mirror surface assembly to concentrate the rays and create an intense beam of energy. Such assemblies generally comprise a parabolic reflector or lens assembly to reflect sunlight onto a focal receiver, usually in the form of a pipe carrying a thermal fluid capable of absorbing heat (such as highly refined paraffinic petroleum oil or molten salt), which is pumped to a heat exchanger that boils water. The boiling of water produces steam that drives turbines which generate electricity.
Two common forms of concentrating type solar collectors are parabolic trough collectors and linear Fresnel collectors. A known parabolic trough collector 10 is shown in FIG. 1. Sunlight is reflected from a curved mirror surface 12, generally of a parabolic shape, to a receiver 14 (containing a heat absorbing medium) positioned at the focus (or focal axis) of the mirror surface 12. The mirror surface 12 and supporting structure is aligned on a north-south or east-west axis and rotates to track the daily motion of the sun. Commercial parabolic trough collector sites have mirror apertures of about 5 to 6 meters in width and are typically many hundreds of meters in length.
However, a problem with existing parabolic trough collectors utilising curved mirror glass is that the curved mirror is difficult and expensive to manufacture. Another problem with parabolic trough collectors is that the structure is heavy and cumbersome, requires strict design and manufacturing tolerances, typically requires complex rotating joints for the pipe carrying the thermal fluid, and is energy intensive to operate.
A known linear Fresnel collector 16 is shown in FIG. 2, in which sunlight is collected by an array 18 of individual reflectors, all of which are identical to a reflector 20, which is shown in more detail in FIG. 3. The array 18 is aligned in a north-south or east-west axis. Reflector 20 comprises a slightly concavely curved mirror 22 which is mounted on a frame 24 pivotable on rollers 26 and 28 connected to a drive system. Alternative designs of linear Fresnel collector are also known, but all such designs share the essential attribute that, as shown in FIG. 4, each reflector must rotate individually and independently over the course of a day to reflect the sunlight to a focal receiver 30 containing a heat absorbing medium.
A problem with known linear Fresnel collectors is that they need individual drive and control and positioning systems for each reflector, and complex supporting structure, which adds considerably to the overall complexity, vulnerability to failure, and cost of the collector. Another problem with known linear Fresnel collectors is that adjacent reflectors shade each other at low sun angles, such as at dawn or sunset, which decreases the overall efficiency of the collector.
The overall efficiency of linear Fresnel collectors is further decreased because of the relatively longer focal length required when compared to a parabolic trough collector of equivalent aperture width, and subsequent reflection losses at the remote edges of the mirror surface, particularly when the sun is at low angles. Sunlight striking the mirror surface at an acute angle reduces the amount of sunlight reflected upon the focal receiver, which approaches zero as the sun approaches the horizon.
The closest known prior art revealed in searches conducted to date are hereinafter described.
Japanese Patent Application No. 55112956A discloses a system utilising refracting focal lenses and no reflection in involved. The axis of rotation of that system is an axis around the centre line of the main tube, rather than an axis that is located below the main tube (or receiver). Also, the tube (or receiver) is not elevated from the surfaces where focusing of light occurs in that system.
US Patent Application No. 2009/0314325 discloses a system having many focal points for its many parabolic reflector strips in each reflector module, rather than having a common elevated receiver to which incident solar radiation from the rows of reflectors is reflected. Also, each truss in that system supports a plurality of reflector modules, and each such truss rotates relative to the support frame. There is no fixed angle of the reflector modules relative to the support frame in that system, and the rotation axis of the support frame is perpendicular to the longitudinal axis of each truss. This allows the reflector modules to move relative to the support structure.
US Patent Application No. 2009/0174957 discloses a system that utilises a plurality of planar reflecting mirrors which can be adjusted to different respective angles on the basis of the alignment of each mirror with an electric field, and no elevated and pivotable support structure for a fixed array of reflectors and a receiver is disclosed.
US Patent Application No. 2009/0272425 discloses a system utilizing a parabolic main reflector, with the Fresnel lens only being used for refraction, not reflection.
It is an object of the present invention to provide a solar energy collector system that overcomes, or substantially ameliorates, the aforementioned shortcomings of the prior art, or at least provides a useful alternative.
It is another object of the present invention to provide a solar energy collector system which is relatively light, does not require a complicated drive system, and is simple, versatile, cheap and easy to manufacture and erect compared to prior art solar collectors.